The present invention relates to an apparatus for optically reproducing information recorded at high density on a recording medium by application of light beam spots thereto and detection of light returning therefrom.
Various types of optical reproducing apparatus have been devised hitherto. One known arrangement is to apply a first light beam spot to a recording medium for reading information recorded thereon and further to irradiate second and third light beam spots thereto for track following control.
Generally, the diameter of a light beam spot applied to an information track of a recording medium depends on the wavelength of light emitted from a light source such as laser and the numerical aperture (N.A) of a condensing lens employed therefor. In order to accurately read information signals from a target track without crosstalk resulting from reproduction of adjacent track information, it will be required that the information tracks are formed such that the distance therebetween (track separation) is greater than the diameter of a light beam spot determined in accordance with the wavelength and the numerical aperture.
In optical reproducing apparatus currently used, the wavelength of laser beam is 780 nm and the numerical aperture of a condensing lens is 0.5, whereby the diameter of a beam spot applied to a recording medium becomes 1.28 microms in minimum. Therefore, the track separation is set to 1.6 microns in consideration of prevention of crosstalk, resulting in one hour recording of NTSC signal per one side of a recording medium having 30 cm diameter in the case that the minimum pit length is 0.56 microns. An important problem in the recording/reproducing art relates to the need for a recording medium which can be formed with higher recording capacity. However, the prior art techniques do not produce satisfactory results for meeting requirements in terms of high density recording because difficulty is encountered to achieve the reduction of the distance between information tracks which in turn causes generation of crosstalk.